1. Field of the Invention
The present invention relates to a brake system and method for a two-wheeled motor vehicle. More particularly, the present invention relates to a brake system and method for a two-wheeled motor vehicle which is operable to coordinate a front wheel braking operation and a rear wheel braking operation at a proper output ratio for a given situation, depending on driving conditions and braking operations.
2. Background Art
A brake system for a two-wheeled motor vehicle may employ a combined brake system (CBS: COMBINED BRAKE SYSTEM; hereinafter referred to as “CBS”) in which when a braking operation unit (lever or pedal) at either of a front wheel and a rear wheel is operated, hydraulic pressure is applied to wheel braking means for the wheel associated with the braking operation unit which has been operated. At the same time, hydraulic pressure is also applied, in an appropriate ratio, to wheel braking means for the other wheel for which the braking operation unit has not been operated.
Heretofore, a brake system employing CBS has been disclosed, such as for example, in Japanese Laid-Open Patent No. Hei 7 (1995)-196068.
In this brake system, a master cylinder synchronized with a brake lever (braking operation unit) at a front wheel is connected with a wheel braking means for the front wheel through a hydraulic passage; a second master cylinder which generates hydraulic pressure by use of a brake torque is provided at the wheel braking means at the front wheel; the second master cylinder and a wheel braking means for a rear wheel are connected with each other through another passage; and further a control valve, which controls the distribution ratio of hydraulic pressures to the front and rear wheels in response to a state of braking operation, is provided on the hydraulic passage.
The control valve is uniquely determined in a manner that the distribution ratio of hydraulic pressure for the front and rear wheels corresponds to an input hydraulic pressure (braking force). To be specific, as shown in FIG. 13(b), when a braking for the front wheel is gradually increased, a braking force for the rear wheel is proportionally increased until the braking force for the front wheel attains a setting value. The braking force for the rear wheel is thereafter held constant, and decreased in an inversely proportional manner when the braking force for the front wheel goes beyond a next setting value.
Accordingly, in this brake system, a braking efficiency is caused to increase by increasing the distribution of braking force to the rear wheel, while braking on the front wheel is operated within a certain defined range. When the brake at the front wheel is operated out of the range, the braking force on the rear wheel is gradually decreased, hence coping with reduction of a rear wheel contact load where the rear wheel is in contact with the ground.
However, in this known brake, a distribution ratio of hydraulic pressure for front and rear wheels is uniquely determined for an input hydraulic pressure (braking force) for the front wheel. Therefore, some operations may occur which are not preferred by a rider. The operations depend on a very limited environment for use of a vehicle, riding conditions, and the like. Hence, modifications by taking into account this problem are expected.
Since the above-described known brake system is uniquely determined in that the distribution ratio of hydraulic pressure for the front and rear wheels corresponds to an input hydraulic pressure for the front wheel, the distribution ratio of hydraulic pressure takes the same ratio, as shown in FIG. 13(b), not only when the braking force for the front wheel is increased, but also when the braking force for the front wheel is decreased. However, in the case where the front wheel is subjected to a braking force which is greater than a setting value, when the braking force for the front wheel is increased, the braking force for the rear wheel is gradually decreased along a characteristic slope. This means that when the braking force for the front wheel is decreased from this state, the braking force for the rear wheel is gradually increased along the same characteristic slope. For this reason, a rider who is used to riding a two-wheeled motor vehicle without CBS may be uncomfortable with the braking feel at the time of braking as described above.